Blender 2.9 weight-painting only selecting vertices rather than bones?
In order to select bones/vertex groups with Ctrl click while in weight paint mode, you have to select the rig in object mode, then shift select the mesh, then switch to weight paint mode.
If you can select only vertices and the paint isn't working, probably you have activated one of theese little buttons, which limit the action of the brush to selected vertices/faces only.